1. Field of the Invention
This invention relates to a novel organic compound and a liquid crystal composition containing the same. More particularly it relates to an organic compound having an optically active group and a liquid crystal composition containing the same.
2. Description of the Related Art
Use applications of liquid crystal display elements have been rapidly expanded due to improvements in circuit, driving mode and cell preparation technique and further, particularly due to improvement in the characteristics of liquid crystal compositions filled in the elements. However, as to liquid crystal display elements, a number of problems to be solved have still been left.
For example, narrow angle of view, inferior contrast, low response rate, still small display capacity, reduction in the display quality due to ambient temperature change, etc. are mentioned. Among these, the reduction in the display quality due to ambient temperature change is caused by temperature change in the threshold voltage Vth.
In recent years, as a means usually employed, there is a method of retaining the display quality by adding a slight quantity of an optically active substance to a liquid crystal composition to inhibit the reverse twist of liquid crystal molecules and thereby impart a right turn or left turn helical structure to the liquid crystal molecules. For example, this method can have an advantageous effect upon the cases of TN (twisted nematic) mode display elements, SBE (supertwisted birefringence effect) mode display elements, etc.
However, if the twistability of an optically active substance added as a dopant is too low, it is required to add the dopant in a relatively high concentration in order to obtain a necessary pitch; hence it is evident this will have a disadvantageous effect upon other substance parameters.
Thus, there has been long awaited an optically active substance having a high twistability i.e,. a capability of exhibiting, when added to liquid crystals, a shorter pitch than those of other substances.
For example, when a commercially available CB-15 made by BDH Co., Ltd. or compounds disclosed in Japanese patent application laid-open Nos. Sho 62-81354/1987 and Sho 62-81355/1987 are added in only one % by weight to nematic liquid crystal compositions, they have a capability of exhibiting a pitch around 10 .mu.m; hence they can be said in a certain sense to be practically usable, optically active substances.
However, when generally known optically active substances, including the above-mentioned compounds, are added, they increase the pitch of liquid crystal substances with temperature rise; hence they often have an undesirable influence upon the liquid crystal substances. For example, in the case of SBE mode, the intrinsic pitch P of liquid crystal compositions varies depending on temperature change, whereby the ratio (P/d) of the intrinsic pitch P of liquid crystal compositions to the cell thickness d of display elements also varies. Usually the P/d is 2 or less, but if it exceeds 2 due to temperature change, 270.degree. twist changes to 90.degree. twist.
Further, in the aspect of improvement for increasing the display capacity, it is necessary to improve the steepness of change in the transmittance when voltage is impressed to display elements. G. Bauer and W. Fehlenbach reported a calculation result that when the twist is changed to 270.degree. , the steepness is improved to a large extent (15th Freiburg Liquid Crystal Meeting (1985)), but in this case, too, it is necessary therefor to be free from change in the intrinsic viscosity depending on temperature (i.e. temperature-dependency of intrinsic pitch).
As to a method for solving this problem, an optically active compound having a negative temperature characteristic i.e., a substance the intrinsic pitch of which decreases with temperature rise, has been found in recent years, and when this substance is mixed in a suitable quantity with a conventional optically active substance having a positive temperature characteristic, a composition having no temperature-dependency of the intrinsic pitch has been obtained (Emoto et al, Japanese patent application No. Sho 61-179194/1986).
However, such a method wherein a compound having a positive temperature characteristic is mixed with a compound having a negative one in a suitable proportion to be free from the temperature-dependency of the intrinsic pitch is very troublesome, and further it is difficult to have a pitch of a necessary length precisely exhibited.
In order to solve these problems, a compound having a short pitch and also having no temperature-dependency of the intrinsic pitch has been desired.